The drug reduces the pleasure from smoking - the classic "nicotine buzz" - by producing antibodies that stick to nicotine molecules and stop them entering the brain.

Each jab lasts about a month. Now 300 people are being given repeat doses to see if the effect will last months and help them give up altogether.

If the trials are successful the drug NicVax could be on the market in three years.

The trial is being carried out by scientists at the University of California, San Francisco, and eight other institutions.

Lead researcher Dr Victor Reus, professor of psychiatry at the university, said one key question will be whether booster shots could help prevent people who have given up from relapsing.

The US Food and Drug Administration has put the trial on fast-track for approval.

Dr Frank Vocci, a director of the US National Institute on Drug Abuse, said: "That means they will do a rapid review and get back to the sponsor within months. They have put a spotlight on it and really tried to move it along because it would be a unique product."

Thomas Rathjen, a spokesman for NicVax developers Nabi Biopharmaceuticals, said the results of the study should be available next year.